That's the whole thing...if Lukaku goes somewhere else, that club is surely going to have a talent that is getting blocked. We've suddenly got a ton of money to replace him. That being said, the incremental difference between Martial and Lukaku is not $100M. So no club would be nuts enough to hold on to both.
The other question is, which of those big clubs wants Rom more than cheaper options that already probably have better reps? Morata isn't going to cost $100M. MBappe isn't close to $100M, either. Griezmann isn't a pure goal scorer, but is he going to be $100M? He'll be expense, but he's probably more highly regarded than Rom. Alexi Sanchez could be available. As could Costa, Aguero, although likely not. Belotti is out there, as is Ibrah.
There aren't that many big time clubs that are ALWAYS in the CL that also have the money to sign Rom. He may just find out - as he likely is now - that his best chance for consistent minutes and European football is at Everton. Maybe not, but Everton still has no reason to sell this summer unless they are blown away with a huge number. And I'm sure we wouldn't promise him he could move unless we got that number.